Gas turbine engine disks commonly have slots for attaching blades which are generally axially oriented. These slots have a profile which mates with the roots of the blades, and have a configuration which will retain the blades in the slots under the applied centrifugal forces incurred in operation of the engine. The slot profiles are often of a "fir-tree" configuration to increase the load bearing area in the slot, although other configurations are also employed.
The almost universally employed method for making these slots in the disks is broaching, a process in which a series of progressively differently shaped cutters is mounted on a rack which is pulled through the workpiece. Each cutter is slightly different from the previous cutter, and is designed to gradually remove material in a manner which eventually creates the finished profile of the slot.
As gas turbine engines have become larger and more powerful, the slots in the disks have become larger and deeper, and the materials employed have become tougher and more difficult to machine. Broaching of the slots requires a greater number of cutters, and consequently a greater length of travel for the cutter bars in order to remove the required amount of material. In many instances, the cutter bar length requirement is greater than the capacity of the machines on hand, and to add new capability sufficient to meet the new requirements would be cost-prohibitive.
It has also been found that, with some materials which work harden, the material can be hardened sufficiently during the early stages of the broaching operation that cutter wear becomes excessive on the later, finishing cutters. This can be very detrimental to surface finish in the slots. In some cases, the forces generated during the later broaching stages are so high that the material on either side of the slot is deflected, resulting in dimensional deviations in the slot shape, or in extreme cases, the material separating the slots can be permanently deformed.
The setup of a broaching machine for the machining of a particular part can be a time-consuming and costly operation. Since the broaching of the slots in a disk usually requires the entire capacity of the machine, it is necessary to tear down and re-tool a broaching machine every time a part having a different slot configuration is to be machined. These factors work in opposition to the current trends toward "just-in-time" delivery of component parts, and to the processing of small lots of parts in response to short model runs.
Our invention (1) provides a process for making blade attachment slots which reduces the dependence on broaching, (2) reduces the time required to machine the slots, and (3) increases the flexibility of the operation.